This invention relates to a thermopile sensor and in particular to a radiation thermometer or a motion detector with a thermopile sensor.
From U.S. Pat. No. 4,722,612 an infrared thermometer having thermopile devices is known. The thermopile device is mounted on a disk-shaped insulative sheet, with the hot junctions being arranged in the center of the sheet and being surrounded by the cold junctions in the form of a circle. The sheet is stretched across a thermally conductive substrate of toroidal shape, with the thermopile being located on the upper side, and the substrate on the underside, of the sheet. The hot junctions of the thermopile lie in the area of the central aperture of the toroidal substrate, whilst the cold junctions of the thermopile lie in that area of the sheet that is supported by the substrate. As a consequence, in the known thermopile the cold junctions are thermally coupled to the substrate much better than the hot junctions and hence have a thermal capacity exceeding that of the hot junctions by a multiple.
When the known infrared thermometer is used under variable ambient temperature conditions, a temperature difference occurs between the housing of the infrared thermometer and the thermopile sensor, causing the heat radiation of the thermometer housing to superimpose itself upon the heat radiation to be measured, corrupting the measurement result. In addition, a temperature difference then occurs also in the interior of the thermopile sensor because, naturally, first the housing of the thermopile sensor experiences a temperature variation due to thermal conduction and/or convection which then propagates inwardly to the thermopile. For example, when the housing of the thermopile sensor is heated by the housing of the infrared thermometer, the sensor housing heats the air in the interior of the thermopile sensor very rapidly, which however in turn heats only the hot junctions of the thermopile by convection. In the known thermopile sensor the cold junctions are strongly thermally coupled to the substrate which, by virtue of its high thermal capacity, is practically not heated by the air. Naturally, however, also the substrate and with it the cold junctions are heated, but this occurs comparatively slowly by heat conduction from the housing of the thermopile sensor. Therefore, the temperatures of the cold and hot junctions heat at different rates, that is, temperature gradients occur also within the thermopile, introducing errors in the temperature measurement process. To compensate for such errors, the known infrared thermometer includes two thermopile devices of like construction, whereof one is exposed to the radiation to be measured while the other is not, and which are connected in series opposition to each other.